Position a rack in the center of the oven and heat the oven to 475° F. Line a heavy-duty rimmed baking sheet with parchment. In a medium bowl, toss the sweet potatoes with enough of the olive oil to coat generously, the salt, and a few grinds of pepper.
Turn the sweet potatoes out onto the baking sheet and arrange the pieces so that they are evenly spaced and lying on a cut side. If the pieces cover the baking sheet sparsely, arrange them toward the edges of the baking sheet for the best browning. Roast the sweet potatoes until lightly browned on bottom, 10 minutes. Flip and continue to roast until tender, 5 to 10 minutes.
Return the sweet potatoes to the bowl in which you tossed them with the oil, or put them in a clean serving bowl. If they seem a bit dry, drizzle them with a little oil. Season to taste with salt, pepper, and lemon juice, if using.